Have you ever wondered why a carburetor needs an accelerator pump? And, more importantly, how this small mechanism prevents the infamous “flat spot” during acceleration?
The principle is actually quite simple.
What is the purpose of the accelerator pump?
When you open the carburetor throttles quickly, the airflow increases immediately. However, the fuel does not respond instantly to this change in airflow. This can temporarily produce a mixture that is too lean, causing hesitation, a stumble, or a “flat spot” during acceleration.
That is precisely the job of the accelerator pump: to provide a small additional amount of fuel immediately when the throttles are opened.
How it works, step by step
When the throttles are closed or returning toward their idle position, lever 25 (pump cam) acts, through linkage 27 (PAF10410), on the pump piston 26 (pump piston).
The piston then moves upward in its cylinder. This upward movement creates a vacuum that draws fuel from the float bowl into the pump cylinder through the inlet valve 23 (inlet and discharge valve).
The pump is therefore “recharged”, ready to operate during the next acceleration.
When the throttles open, the geometry of the mechanism allows the control rod 27 (pump control rod) to release the piston. The piston is then pushed downward by its spring 28 (pump control rod spring).
The fuel contained in the pump cylinder is compressed and forced through the passages and the discharge valve 21 (pump valve components PAF5830000300 - PAF5273000100 - PAF6101500600). It then passes through the accelerator pump jets 24 (accelerator pump jet) before being injected directly into the carburetor throats.
This additional fuel is injected immediately as the throttles open. It is what gives the engine a crisp, progressive response and prevents the momentary lean condition.
What about excess fuel?
The inlet valve (23) also performs another function. It contains a calibrated orifice that allows some of the fuel delivered in excess by the accelerator pump to return to the float bowl.
The actual amount of fuel injected into the engine therefore depends, among other things, on the calibration of this assembly and on the accelerator pump jets.
In summary
The operation can be summarized very simply:
Throttles closed → piston moves up → pump fills with fuel.
Throttles opened → spring pushes piston down → fuel is delivered through the accelerator pump jets → engine immediately receives the additional fuel it needs.
It is this ingenious, entirely mechanical system that allows Weber DCOE carburetors to provide an immediate throttle response and eliminate the dreaded flat spot during acceleration.
As always with a Weber carburetor, correct accelerator-pump adjustment is essential: piston stroke, jet size, fuel delivery, and progression must all be matched to the engine and its intended use.
Are you looking for main jets, emulsion tubes, air-correction jets, or a Weber DCOE rebuild kit?
Check out the selection of genuine Weber parts and find the components suited to your Weber DCOE or DCO/SP carburetor:
Weber DCOE parts
Weber DCO/SP parts
See also our Weber DCOE guides:
- Weber DCOE Idle Circuit — to understand how fuel delivery changes as engine speed increases.
- Weber DCOE Main Circuit
PAFClassic 🇬🇧 is an official distributor of WEBER and WEBCON products.
All the carburetors we offer are genuine WEBER carburetors manufactured at the Magneti Marelli licensed production facility in Spain.
We have access to more than 11,000 references covering carburetors and spare parts, as well as the WEBER technical library, allowing us to supply the components suited to your carburetor.
Always make sure you are purchasing genuine WEBER products. Copies of these legendary carburetors are now widespread on the market. Although they are often offered at attractive prices, they can have inferior manufacturing quality and may not be compatible with genuine WEBER components. Differences in jet dimensions, internal components, and calibration can make correct tuning considerably more difficult.
